As a technique of implementing a wide observation width and a nigh resolution of a synthetic aperture radar (SAR), there is a multi-beam SAR in which a plurality of receivers are disposed in a radar device that observes a target. This multi-beam SAR can cope with both a wide observation width according to a plurality of signal series at a low pulse repetition frequency (PRE) and increase in the resolution in an azimuth direction according to sampling in a high direction. By using a restoration algorithm for this multi-beam SAR to extend the Doppler frequency bandwidth of each signal series, the conditions which are required for the design of the multi-beam SAR can be eased (for example, refer to nonpatent reference 1).